U.S. Pat. No. 3,291,467 describes a laundry dryer which is equipped with a gas-heated heating device for heating the process air stream. The burner of the heating device is disposed upstream of the inlet opening of a heating channel. The heating channel is inclined upwardly so as to follow the natural orientation of the flame. In the burner of this laundry dryer, primary air is mixed with the gas as combustion air upstream of the burner surface, and secondary air is added at the inlet of the heating channel. At the outlet of the heating channel, tertiary air is added to the hot gas and passed as a process air stream through the drum.
In this burner, combustible gas and air are partially premixed. The so-called “primary air” is drawn in by the injector effect of the gas nozzles. The secondary air portion and the tertiary air portion are drawn in by the process air fan. The secondary air enters the heating channel at the burner head, forming an envelope around the flame so as to prevent contact of the flame with the walls of the heating channel or combustion chamber, and thus to prevent heat transfer to the housing. The tertiary air enters at the end of the flame through holes in the shell of the heating channel. The tertiary air holes are arranged in the lower half-shell relative to the cross section of the heating channel, thereby preventing contact of the flame with the wall of the heating channel. The tertiary air further serves to reduce the temperature of the hot air at the entrance to the process air duct to the temperature of the process air. The gas supply is controlled by a valve and a pilot-flame- or spark-monitoring control system. It has been found that in spite of the constructional features, such as the upwardly inclined orientation of the heating channel and the arrangement of the tertiary air supply in the lower half-shell in the region of the flame end, it is still not possible to achieve a combustion with particularly low pollutant emissions.
German documents DE 103 32 338 A1 and DE 103 32 339 A1 describe a heating device for a laundry dryer, which has a first heating channel and a second heating channel. Both heating channels are circular in cross section, the second heating channel having an enlarged cross section compared to the first heating channel. The head of the burner projects into the inlet opening of the first heating channel, the axial centerline of the burner being located below the centerline of the first heating channel. The outlet opening of the first heating channel projects eccentrically into the inlet opening of the second heating channel in downwardly offset relationship therewith. Due to this feature, the inlet opening for the tertiary air is formed principally at the upper side of the second heating channel. In this manner, the hot gases, which issue from the first heating channel and which flow upwardly due to thermal buoyancy, can better mix with the tertiary air in the upper region of the second heating channel. The first heating channel further has a guide device for secondary air which enters through the inlet opening, said guide device extending at a downward slope in flow direction from a middle upper portion of the first heating channel to the outlet opening thereof. This feature reduces the thermal buoyancy of the flame, causing the flame to be directed downwardly at the end of the first heating channel. This results in a combustion with reduced pollutant and noise emissions.
A similar gas-heated laundry dryer having a heating device is described in U.S. Pat. No. 3,826,607. In that disclosure, there are also provided two heating channels, the outlet opening of the first heating channel, into which opens the flame opening of the burner, having a smaller cross section than the inlet opening of the second channel, and the outlet opening of the first channel projecting into the second channel.
However, the construction including a first heating channel and a second heating channel requires greater effort during manufacture and during installation of the heating device in a laundry dryer. The separate subassembly formed by the first and second heating channels must be accurately aligned with respect to the burner during assembly in order to obtain the desired air flow.